Hot melt adhesives typically exist as solid masses at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing of a variety of disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, sanitary napkins, pantyshields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate. A second substrate is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesive is the lack of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating costly processes associated with liquid carrier removal.
For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film through a slot die by using piston or gear pump equipment. In these cases, the substrate is brought into intimate contact with a hot die under pressure. The temperature of the die must be maintained well above the melting point of the adhesive, typically in the range of 150 to 200.degree. C. For some applications, particularly for manufacturing nonwoven articles, bonding of delicate and heat sensitive substrates, such as thin gauge polyethylene film, is often involved. Direct contact between the substrate and the die, in these cases, must be avoided to prevent the substrate from burning or distorting. Several application methods have been developed through which a hot melt adhesive can be spray coated with the aid of compressed air onto the substrate from a distance. These indirect techniques include spiral spray, and various forms of melt-blown methods. Direct contact between the coating head and the substrate is thus eliminated. All the coating techniques mentioned above are well known to those skilled in the art and commercial equipment is readily available.
The indirect coating techniques, however, pose stringent requirements on hot melt adhesives. The viscosity of the adhesive must be sufficiently low, usually in the range of 2,000 to 30,000 cP, preferably in the range of 2,000 to 15,000 cP, at the application temperature. Many other physical factors, especially the rheological properties of the adhesive, come into play in determining the sprayability of a hot melt adhesive. The majority of commercial hot melt products do not lend themselves to spray applications. There are no widely accepted theoretical models or guidelines to predict sprayability and it must be determined empirically with application equipment.
In accordance with the present invention, it has been found that a blend comprising a flexible polyolefin (FPO), a tackifying resin, a plasticizer and, optionally, a synthetic polyolefin wax or petroleum wax provides a sprayable hot melt adhesive which has novel combinations of properties including good adhesion to a variety of substrates, low viscosity, good heat stability and oil/ointment resistance. The composition of the present invention is particularly useful in manufacturing nonwoven articles for binding of polyethylene and polypropylene films, nonwoven fabrics and elastic bands to each other or to themselves.
The flexible polyolefin polymers useful in this invention are essentially high molecular weight propylene homopolymers or copolymers of propylene with other a-olefin monomers such as ethylene, butene-1 or hexene-1. FPOs should not be confused with the conventional crystalline polypropylene and amorphous poly-.alpha.-olefins (APAO). It is well know to those skilled in the art that the conventional crystalline polypropylene are high molecular weight polymers of propylene with a predominantly isotactic chain structure. The isotactic configuration can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer. This type of stereochemical chain structure can be illustrated graphically by using the Fischer projection formula as follows: ##STR1##
Due to its high degree of chain regularity, the conventional isotactic polypropylene (IPP) is highly crystalline with crystallinity usually greater than 50% and a heat of fusion, which is a measure of crystallinity, greater than 70 J/g. The conventional crystalline polypropylenes are usually stiff materials having high density and a high melting point. They have not been used as the sole polymer base for hot melt adhesive applications. Typical conventional IPP usually has a melt flow rate, which is inversely related to average molecular weight, in the range of 0.5-200 g/10 min. as measured in accordance with ASTM D-1238 method.
APAOs, on the other hand, are a family of essentially amorphous low molecular weight homopolymers of propylene or copolymers of propylene with ethylene, butene-1 or hexene-1. In contrast to the regular isotactic structure, APAOs are atactic with the methyl groups on the successive monomeric units sterically randomly distributed on the opposite sides of the hypothetical plane through the polymer chain. The stereo configuration of atactic APAO can be depicted by using the Fischer projection formula as follows: ##STR2##
The irregular stereo configuration hinders the formation of any ordered three-dimensional array and as a result, amorphous poly-.alpha.-olefins, as the name indicates, are essentially noncrystalline, or amorphous soft materials having low mechanical strength and low density. Compared with crystalline polypropylene, APAOs are usually low average molecular weight polymers having a melt flow rate of around 2000 g/10 min. as measured in accordance with ASTM-D-1238.
Flexible polyolefins (FPO) is another unique family of propylene-based polymers. In contrary to the predominantly isotactic chain configuration of IPP and predominantly atactic chain configuration of APAO, the stereostructure of FPO can be described as having segments or blocks of regular isotactic structure that are interspersed by segments or blocks of atactic structure. Due to this unique molecular chain architecture, FPOs are semi-crystalline with crystallinity and melting point below those of IPP. The unique molecular structure of FPO leads to an unusual and desirable combination of physical and mechanical properties such as low density, low melting point, flexibility, softness and elasticity.
In addition to the difference in molecular structure, FPOs are also readily distinguishable from IPPs and APAOs by their unique physical properties. Typical FPOs will have a melting point between 250-320.degree. F. and a heat of fusion in the range of 15 to 60 J/g, whereas crystalline IPPs usually have a melting point about 340.degree. F. and a heat of fusion above 70 J/g. APAOs, on the other hand, are usually predominantly amorphous without a well-defined melting point although some commercial APAO products may exhibit very low degree of crystallinity with a heat of fusion less than 10 J/g. Other profound differences between FPO, IPP and APAO lies in their densities. The density of FPO is typically between 0.87 to 0.90 g/cm.sup.3, which is in between those of IPP and APAO. IPPs have the highest density ranging from 0.90 to 0.95 g/cm.sup.3 and APAOs, the lowest ranging from 0.85 to 0.87 g/cm.sup.2. The differences in physical properties among those three unique families of polyolefins are well known and have been the subject of many articles.
Due to their high melting point, high degrees of crystallinity and the lack of desirable physical and mechanical attributes such as flexibility, elasticity and softness, the conventional IPPs have not been used alone as the polymer base for hot melt adhesive applications. A hot melt adhesive based on IPP would be too brittle to yield acceptable bond strength and yet would require high application temperature well beyond the melting point of 340.degree. F. for IPP.
Hot melt adhesives containing APAO, on the other hand, are known in the art. These adhesives typically have greater than 50% by weight of the polymer. It is well known that adhesives based on APAOs generally have poor cohesive strength, poor heat resistance, low elevated temperature peel and low shear values. APAOs have not found much use in disposable nonwoven applications where a combination of high bond strength at very low coating weight and easy processability by spray techniques mentioned above is required. APAO based adhesives usually lack such capability.
For example, Ryan discloses in U.S. Pat. No. 5,747,573 an APAO based hot melt adhesive composition useful for bonding plastics and metallized foil containers. The adhesive composition contains a blend of APAO, a solid benzoate plasticizer and a hydrocarbon tackifier.
Trotter et al, U.S. Pat. No. 4,022,728 describes a hot melt pressure sensitive composition comprising a blend of APAOs, a low molecular weight substantially amorphous elastomer, a liquid tackifier and a crystalline polypropylene (IPP) in an amount up to 2% by weight. It is claimed that the composition provides good adhesive properties at low temperatures.
Kehr et al, U.S. Pat. No. 5,185,398, discloses an adhesive coating composition comprising 90-99.9 parts by weight of an olefin-carboxylic acid/acid derivative polymer carrying functional groups. The composition is claimed to have major improvement in adhesion to polyolefin plastics and metals over adhesives based on APAO and tackifier blends.
Meyer et al, U.S. Pat. No. 4,120,916, discloses hot melt adhesive compositions comprising a blend of polyethylene, APAO and crystalline propylene containing polymer. These hot melt compositions are said to have a novel combination of properties such as short hot tack times and open times for the bonding of paraffin modified corrugated board.
Lakshmanan et al, U.S. Pat. No. 4,761,450, disclose a compatible polymer blend useful as hot melt adhesive comprising a low density ethylene polymer, a copolymer of butene-1 with ethylene or propylene, a hydrocarbon tackifier and a low molecular weight polymer selected from the group consisting of a low molecular weight liquid polybutene, an amorphous polypropylene and mixtures thereof.
It is apparent that all the above prior art adhesive compositions are based on the APAO family. As noted above, APAOs differ significantly from FPOs used in the present invention in both molecular structure, average molecular weight, physical and mechanical properties. These prior art APAO adhesives are formulated for applications other than for disposable nonwovens products and usually lack sprayability.
In the construction of disposable nonwoven articles such as diapers, hot melt adhesives based on styrenic block copolymers such as styrene-isoprene-styrene (SIS) block copolymers or styrene-butadiene- styrene (SBS) block copolymers are widely used to bond polyethylene film, or the like, to tissue or nonwoven substrates. The block copolymer based adhesives are particularly useful in the construction of inner leg gather or cuff which is employed to prevent leakage of bodily waste from around the user's legs. During use, this cuff or flap is held in place with one or more elastic bands surrounding the leg. These elastic bands are typically held in place and attached to the disposable article by hot melt adhesive.
These block copolymer adhesives, however, possess shortcomings such as viscosity instability which manifests itself at elevated temperature. Another shortcoming is that these block copolymers lose most of their bond strength upon exposure to mineral oil or other oil based ointments. Mineral oil and other oil based ointments are often used on infants to treat skin rashes, and thus prior hot melt adhesive compositions, upon exposure thereto, experience adhesive bond failure. As a result, the elastic leg bands may actually let loose from the diaper resulting to complete failure and break down of the inner leg cuff. Therefore, an adhesive that is capable of withstanding exposure to mineral oil or other oil based ointments while still providing sufficient bond strength for elastic band attachment in the inner leg cuff would be highly desirable.